Traditionally drilling holes into and through high strength rock types has been most economically performed by percussive drilling systems. These systems fall into one of two categories; either those where the percussion mechanism is located out of the hole (top hammer systems), or those where the percussion mechanism is located in the hole (DTH systems). Top hammer systems require the use of a string of percussion drill rods to transmit force to the rock face. The transmission of percussion shock waves through a series of rods creates limitations as to hole depth and/or drilling accuracy, especially in larger hole sizes, as well as reliability issues. DTH drilling solves the problems associated with top hammer systems by creating the percussion shock waves at the bottom of the hole, where they act directly on the drill ‘bit’ in contact with the rock. Such DTH systems have traditionally been pneumatically powered, using compressed air to transmit energy through the drill rods down the hole to the percussion mechanism at the bottom. Such drilling systems are typically energy inefficient and slow compared to hydraulic top hammer drill systems, especially in smaller hole sizes and/or shallow depths. In an effort to combine the advantages of both top hammer and DTH drilling systems water powered DTH systems have been developed. However these systems have not found widespread use as they suffer from reliability and economic constraints, by using a non-lubricating and potentially corrosive medium (i.e. water) to transmit energy to the percussion mechanism.
EP0233038 and U.S. Pat. No. 5,092,411 disclose the concept of an oil powered DTH drill system. Both of these disclosed drill systems make use of hydraulic hammers fed by external hydraulic hoses clipped into the sides of dedicated drill rods. While the use of an oil powered hammer improves the energy efficiency and reliability of drilling, the arrangements disclosed in these documents suffer from the disadvantage that the external hoses are prone to damage when the hammer is in operation down a hole with resulting unreliability and reduced efficiency in terms of loss of oil and increased operational costs. Operational efficiency is also adversely affected by the complication of reattaching the hydraulic hoses when adding and removing drill rods.
A further source of oil loss with known oil powered drill systems, such as those disclosed in U.S. Pat. No. 5,375,670 and WO96086332 is during coupling and uncoupling of the rods supplying oil under pressure to, and receiving return oil from, the hammer during travel into and out of the drilled hole.
Further loss in efficiency of known hydraulic drill systems, such as that disclosed in JP06313391, can be due to a reduction in impact energy produced and/or reduced cycle speed where the hydraulic accumulator, used to accommodate the varying flow requirements during a cycle of piston extension and retraction, is mounted remotely from the hammer.
A further disadvantage with known hydraulic drill systems is that they are expensive to manufacture and replace when damaged due to the one-piece design of the hammer.
It is an object of the present invention to address the foregoing problems or at least to provide the public with a useful choice.
Further aspects and advantages of the present invention will become apparent from the ensuing description which is given by way of example only.
All references, including any patents or patent applications cited in this specification are hereby incorporated by reference. No admission is made that any reference constitutes prior art. The discussion of the references states what their authors assert, and the applicants reserve the right to challenge the accuracy and pertinence of the cited documents. It will be clearly understood that, although a number of prior art publications are referred to herein; this reference does not constitute an admission that any of these documents form part of the common general knowledge in the art, in Australia or in any other country.
It is acknowledged that the term ‘comprising’ may, under varying jurisdictions, be attributed with either an exclusive or an inclusive meaning. For the purpose of this specification, and unless otherwise noted, the term ‘comprising’ shall have an inclusive meaning—i.e. that it will be taken to mean an inclusion of not only the listed components it directly references, but also other non-specified components or elements. This rationale will also be used when the term ‘comprised’ or ‘comprising’ is used in relation to one or more steps in a method or process.